1. Field of the Invention
The invention relates to a method and an apparatus for coating particles. More precisely, the method and the apparatus are used for the batchwise coating of particles with a coating material which is at least partly liquid during coating.
The particles to be coated may contain, for example, at least one enzyme--such as a protease, lipase or maltase--and, for example, be intended for use as a component of a detergent. Furthermore, it is possible to coat particles which contain at least one pharmaceutical active substance--for example an antibiotic, ibuprofen, theophylline or diclophenac. Moreover, the particles may contain at least one organic acid likewise serving as a pharmaceutical active substance and/or as a nutrient and/or as a nutrient additive--such as citric acid or ascorbic acid--and/or a salt of such an acid. The particles may also contain, for example, at least one crop protection agent.
2. Description of the Prior Art
An apparatus disclosed in German Patent No. 2 030 104 has a vessel and a rotor which is rotatable therein about a vertical axis and has a disk which forms the lower limit of the processing space. An annular gap is present between the wall of the vessel and the edge of the disk. In the vessel, an atomizing nozzle is arranged above the disk. The disk is rotated during operation. The particles are alternately moved outward on the disk and lifted by a process gas flowing upward through the stated annular gap. The moving particles are sprayed from above with a liquid coating material and then dried.
Apparatuses of similar type are also disclosed in U.S. Pat. No. 4,895,733. These last-mentioned apparatuses are used mainly for the agglomeration of particles. However, the possibility of using these apparatuses for coating particles and for this purpose spraying the particles with a coating material by means of at least one atomizing nozzle which is arranged above the bed of the particles resting on the disk is also mentioned.
The coating materials used in the known methods and apparatuses usually consist of solutions and/or dispersions which contain a large amount, typically about 70% to 95% by weight, of liquid solvent and/or dispersing medium during spraying. A substantial disadvantage of the known methods and apparatuses previously described is therefore the fact that a great deal of energy and time are required for drying the sprayed particles, which is usually carried out with the aid of hot air. If the particles contain heat-sensitive substances--such as, for example, enzymes or antibiotics--the drying process must be carried out at an appropriately lower temperature to avoid damaging the said substances and then requires a longer time than when hot air is used. Furthermore, the solvent and/or dispersing medium which is present in the coatings and evaporates during drying of the coatings may give rise to pores and to bulges formed from burst bubbles. At least in the case of small dimensions, porous coatings are therefore permeable, with the result that the sealing and/or protective effect which the coatings are required to achieve in many cases is impaired or can be achieved to some extent only by extremely thick coatings. In addition, the surface roughness caused by the pores and bulges is undesirable in many cases.
In the known methods and apparatuses, water is frequently used as the solvent and/or dispersing medium. However, this may be disadvantageous in the coating of particles containing hydrolyzable substances.
Furthermore, various substances used for the formation of coatings--for example waxes--are soluble not in water but only in organic solvents. However, coating with coating materials which contain organic solvents gives rise to many problems because organic solvents are more or less toxic and are frequently flammable and therefore should not enter the environment.